I do apologize for my delay in reply (I am afraid I was called away to back to back emergencies for two of my own patients).
Now I have to say that I am curious to how your vet knew what bacterial species was present in the urine but only has taken a free catch sample. The reason why we do not use free catch urine samples for culture is because they do not always represent the infection inside the bladder (as the urine is easily contaminated by any bacteria in the vagina or fur that it passes by before collected in a free catch). Therefore, if he is not improving and we are seeing intermittent blood despite their treatment; then I would double check how and if they did their culture to ensure the results they have reported are reliable. And if they suspect an infection and didn't collect a sample via needle/syringe directly from the bladder, then that would be the ideal way to do so to ensure the culture results reflected the infection in the bladder. And once we knew that, we could target any infection properly to clear it.
Otherwise, if we aren't seeing a response to antibiotics and have intermittent bleeding
, then we do have to consider other issues. Just to note, if there is any doubt this is blood, your vet should be able to check a dipstick urinalysis to confirm if it is blood. And if it is, I would note that it'd be ideal to have your vet send a urine sample to the lab to be analysed. The lab can again culture this for infection if need be, but can also examine the urine under the microscope. When doing so, they can check for any signs of crystals (which could cause these signs too but also be a hint of bladder stones being present) or exfoliated cells from any bladder tumors
Finally, we'd also want his vet to check his prostate via rectal examination. Diseases of the prostate (abscesses, overgrowth of the organ, cysts, infections, tumors) can all cause intermittent bleeding into the bladder. (As well, they often cause fecal and urinary passage issues as the prostate can compress both the colon and urethra if it grows larger then it should). Most commonly prostate issues are seen in uncastrated or late castrated dogs, but any elderly male dog with these signs should be checked for this.
Overall, with his lack of improvement, we do have to consider whether the infection diagnosed is what is present and causing him bother. As well, we need to consider these other issues. Therefore, while its good that he has had a scan, we do need to take further steps to rule out these other issues to get to the root of his signs and address for him.
Please take care,